dummynet: Does not depend on ipfw

Allow the dummynet module to be loaded without ipfw, as a first step
towards making pf use it for packet scheduling.

Reviewed by:	donner
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D31903

sys/netpfil/ipfw/ip_dummynet.c

@@ -2735,7 +2735,6 @@ static moduledata_t dummynet_mod = {
 #define	DN_SI_SUB	SI_SUB_PROTO_FIREWALL
 #define	DN_MODEV_ORD	(SI_ORDER_ANY - 128) /* after ipfw */
 DECLARE_MODULE(dummynet, dummynet_mod, DN_SI_SUB, DN_MODEV_ORD);
-MODULE_DEPEND(dummynet, ipfw, 3, 3, 3);
 MODULE_VERSION(dummynet, 3);
 
 /*